The present invention relates to molecular beam epitaxy (MBE) systems.
In molecular beam epitaxy, thin film deposition is achieved by directing molecular beams onto a substrate in an ultra high vacuum. Preferably the beams are not ionized, but are neutral molecular or atomic species, whence the name of the process. The substrate is heated to a temperature where kT is large enough to permit a deposited atom to move laterally for an average distance of at least several angstroms to permit the deposited atoms to find their energetically preferred sites. Thus, MBE permits growth of thin films with extremely high crystalline quality. The MBE technique is generally well known, and has been widely discussed. See, for example, the following review articles, which are hereby incorporated by reference: A. Y. Cho and J. R. Arthur, in Progress in Solid State Chemistry, edited by J. McCaldin and G. Somorjai (Pergamon, New York, 1975), Vol. 10, p. 157; L. L. Chang, in Handbook on Semiconductors, edited by S. P. Keller (North-Holland, Amsterdam, 1980), Vol. 3 Chapter 9; C. E. C. Wood, in Physics of Thin Films, edited by C. Haff and M. Frankcombe (Academic, New York, 1980), Vol. 11, p. 35; C. T. Foxon and B. A. Joyce, in Current Topics in Materials Science, edited by E. Kaldis (North-Holland, Amsterdam, 1981), Vol. 7, Chapter 1.
Molecular beam epitaxy is very attractive as a product technology for many applications due to its unique capabilities. MBE easily produces hetero-epitaxial structures, wherein an epitaxial layer of one material is epitaxially deposited onto an underlying layer of a different material. The abrupt epitaxial transitions which can thus be achieved can be rapidly alternated to achieve superlattice structures, wherein, as the different epitaxial layers become very thin, some anomalous and highly desirable properties appear. Such structures are very difficult to make in any other way. MBE can also be used to make strained superlattices, wherein materials which are not lattice-matched in isolation are nevertheless grown in a perfect epitaxial structure. That is, materials which have the same crystal structure, but which would not have the same lattice spacing normally, can not be grown epitaxially by conventional methods. For example, the lattice constant of InAs.sub.0.4 Sb.sub.0.6 is 0.4% less than that of InAs.sub.0.27 Sb.sub.0.73. Thus, if one attempts to grow an epitaxial layer of InAs.sub.0.4 Sb.sub.0.6 on a InAs.sub.0.27 Sb.sub.0.73 substrate by conventional methods such as chemical vapor deposition, the two lattices would not be matched. That is, it is desirable to have the interface between the two materials preserve the crystalline structure of the materials, so that the first lattice is a smooth continuation of the second lattice, except that more arsenic atoms and fewer antimony atoms are now found on the Group V sites. This can not be achieved by conventional methods, but is readily achieved in superlattice structures by MBE. MBE also promises other unique capabilities, such as epitaxial deposition of insulators over semiconductors, metals over insulators, etc.
However, attractive as these capabilities are for semiconductor device fabrication, MBE systems at present are primarily a laboratory tool rather than a production tool, simply because the throughput of MBE systems is slow. In part, the slow throughput of MBE systems is unavoidable, since it is difficult to achieve good quality deposited material if the deposition rates used are greater than several microns per hour. However, in large part this problem of slow throughput has been due to the difficulties of wafer handling.
Thus, it is an object of the present invention to provide an MBE system having reduced time requirements for wafer handling.
The prior art of MBE systems has used molybdenum substrate holders, with each wafer being soldered to the substrate holder. Due to the extremely high vacuums which are used for growth (typically in the neighborhood of 10-11 Torr), the materials requirements for anything which will be present in the vacuum chamber are extremely stringent. These requirements are especially stringent for items which are in contact with the wafer or are in close proximity to the wafer since contaminants released by these items are particularly likely to be transported to the growth front and be incorporated into the epitaxial layer.
It should be noted that the actual position of the wafer during MBE growth is downward facing. This is done because, even in an ultra high vacuum system, some particulates will inevitably be present, e.g. particulates which adhere to the inner walls of the chamber until released by thermal cycling, accoustic vibrations, or minor mechanical shocks, and flaky arsenic particles from inadvertent arsenic deposition on the cryoshield. At extremely low pressures these particles will not be suspended in the chamber atmosphere, but will simply fall down to the bottom of the chamber. The wafer is positioned facing downward so that it does not intercept the path of any of these falling particulates, and to permit the wafer to face liquid sources such as a liquid gallium source.
It is also well known in the MBE art that the wafer should be rotated during growth. This provides more uniform deposition and heating. In particular, since an MBE system should have multiple molecular beam sources, to take advantage of the flexibility in deposition discussed above, the molecular beams from most of these sources will not impinge on the wafer at right angles to its surface. Essentially all of the depositions are angle depositions, and rotation of the wafer accordingly improves uniformity.
However, the combination of requirements just discussed adds up to a quite different mechanical problem in supporting the wafer during growth. Arc-cast molybdenum is an adequately high-purity material for the substrate holder, and can be machined into complex shapes as desired, but unfortunately the thermal expansion coefficient of molydenum is very different from that of desirable substrate materials, such as gallium arsenide of other Group III-V semiconductor compounds. This means that the substrate must be held in such a manner that the substrate can expand freely during the temperature rise to the growth temperature, and not be strained by the different thermal expansion of the substrate holder.
The conventional method of the prior art to accomplish these objectives is indium soldering to a molybdenum substrate holder. That is, a very pure molybdenum substrate holder is used, which includes pins for attachment to the rotating drive in the growth chamber, and the substrate wafer upon which MBE is to be performed is hand-soldered to the substrate holder. That is, the substrate holder is heated to about 150.degree. C. on a hot plate, and the wafer is manually placed upon it. At this temperature the indium will be liquid, but will freeze again to provide a good mechanical attachment as the wafer is cooled down. The wafer, mounted to a substrate holder, is then loaded into the MBE system. After initial pump down stages as will be discussed below, the wafer and substrate holder are eventually transferred into the growth chamber. In the growth chamber, the wafer is held in a downward-facing position, and heated up to a growth temperature. As the wafer temperature increases above the melting point of indium (about 142.degree. C.), the indium melts, but the wafer is still held in position by the good adhesive properties of the liquid indium. After the growth run is complete, the substrate holder is allowed to cool, so that the indium refreezes and the substrate holder and wafer are then transferred back out of the growth chamber. After the wafer is removed from the MBE system, it is necessary to unsolder it from the substrate holder. This is a very delicate manual step. The same viscosity and adhesion which permit indium to hold the wafer in place during the growth cycle make removal of the wafer from the substrate holder difficult. A further difficulty is that the indium distribution will typically change during growth, especially if the molybdenum holder has been used many times so that the thickness of the indium will often be found to be thicker under the edges of the wafer than under the center of the wafer. After the wafer is removed from the substrate holder, it will typically be found that the indium has partially alloyed with the backside of the GaAs wafer. To avoid producing crystal dislocations the alloyed material must be removed. This is typically done by etching in HCl. To avoid introduction of mobile impurities into the front surface of the wafer during this backside etch, the front surface is typically painted with photoresist prior to this etching step. (Likewise, the indium on the front side of the molybdenum substrate holder can be removed by etching the block in HCl.) However, repeated use of this cleanup etch will gradually erode the molybdenum so that the surface of the substrate holder is no longer planar.
After the indium is etched away, the backside of the wafer is no longer flat. If the wafer is sought to be processed in this condition, very many wafers will be broken during routine handling, since the etched backside will now contain irregularities which can produce local stress maxima. Moreover, use of vacuum chucks in subsequent wafer processing operations now becomes impractical, because the backside of the wafer is no longer flat.
This difficulty can be mitigated by repolishing the backside of the wafer until it is flat again. However, this provides a further manual processing step, in which further wafer breakage is inherently likely. Moreover, if the wafers are not initially of extra thickness, this polishing step will thin them so that they are very fragile and again liable to breakage in routine handling.
Moreover, the front side of the wafer, which now contains the expensive epitaxial layer, must be carefully protected from this further polishing step. A further difficulty with this prior art method is the smoothness of the molybdenum substrate holder. That is, the substrate holder as received (or as fabricated) will not be perfectly smooth, and the irregularities in the substrate holder surface may themselves make removal of the wafer more difficult. This problem can be avoided by polishing the molybdenum substrate holder, but the problem with this is that repeated polishing of the substrate holder will erode it.
All of these extra processing steps required by the indium soldering are very expensive. In particular, any processing step which risks slide breakage after the MBE operation has been performed is extremely expensive since the structure which is thus exposed to a risk of being destroyed may be a structure which has just taken several hours of growth time on a million dollar machine, plus the initial slice cost and the pre-growth slice preparation cost.
It is an object of the present invention to provide an MBE system having a wafer support structure such that the number of handling steps required after growth is minimized.
It is a further object of the present invention to provide an MBE system wherein no unsoldering step is necessary after the growth step.
To achieve these and other objects, the present invention provides an MBE System wherein the wafer on which epitaxial deposition is to occur is not soldered to a substrate holder. Instead, a substrate holder with a lip approximately as high as the thickness of the wafer is used, and a retaining ring attaches to the substrate holder to hold the wafer in place during the growth cycle. The retaining ring, like the substrate holder, is made of high-purity refractory material, such as arc cast molybdenum. The substrate holder and retaining ring are dimensioned to hold the wafer somewhat loosely, to allow for thermal expansion during the cycling up to growth temperature, which is typically about 600.degree. C.
A problem in conventional MBE systems is outgassing of the sources. As shown in FIG. 3, a conventional MBE evaporation source is a small crucuble (in which the source material will be placed), mounted on a vacuum flange together with a resistive heater, a heat shield, and a thermocouple. This structure may contain volatile contaminants, which are likely to escape when the source is heated to the temperatures used for evaporation of the source material.
Therefore, for best quality MBE growth, it has been found desirable to outgass the source, before the source material is actually placed in the crucible, at a temperature of about 1400.degree. C. or higher for at least several dozen hours. After the source material is placed on the crucible, a second bake out step, at about 50.degree. C. over the source evaporation temperature, is performed for a shorter period of about 1 hour. A short exposure to air subsequent to these outgassing steps is not harmful, since these steps are not directed merely at adsorbing water and other low-temperature contaminants, but are directed at removing the high-temperature contaminants which may initially be present in the crucible and in the material of the source structure.
However, while this source outgassing provides better quality grown material, it is obviously quite time consuming. In particular, since the outgassing must be performed under high vacuum conditions, it could be performed with a source in place in the growth chamber of the MBE system, but this would obviously tie up the growth chamber of the MBE system for extended periods and therefore further degrade the already low throughput of the MBE system.
It would be possible to provide a separate high vacuum system for outgassing the sources, but this would obviously be expensive, not only in capital cost but also in technician time, due to the system bake out and other routine maintenance steps which are periodically necessary for any operating ultra high vacuum system.
Thus, it is an object of the present invention to provide a molecular beam epitaxy system which includes means for outgassing sources without degrading throughput of the system.
It is a further object of the present invention to provide a MBE system which incorporates means for outgassing molecular beam sources, without degrading throughput of the system and without requiring any additional vacuum system.
The present invention provides this objective by providing a molecular beam epitaxy system which includes, as is conventional, more than one separate ultra high vacuum chamber. That is, a growth chamber is separated from a sample analysis chamber by a vacuum valve, through which wafers can be passed, and which can be closed to isolate the growth chamber from the analysis chamber. In the present invention, the analysis chamber includes a source outgassing fixture, into which one source can be temporarily attached, so that source outgassing can be performed in the secondary chamber, after a wafer has been loaded into the growth chamber, while growth is proceeding in the growth chamber. Thus, no additional ultra high vacuum facility is required, but source outgassing can be performed with no degradation of throughput.
A further problem with the throughput of prior art molecular beam systems is the periodic chamber bakeouts which are required. That is, in any ultra high vacuum chamber used for MBE growth, it is necessary not only to bake out the chamber walls after the system has been exposed to air, to remove water and other volatile contaminants, but it is also necessary to periodically bake out the system again, even though the system has not been exposed to air, to again purge the system of contaminants. Volatile contaminants such as arsenic and phosphorus may result from the species used during the actual growth period.
This requirement of periodic bake out in ultra high vacuum systems is well known, and the conventional way to accomplish this in MBE systems is to enclose the whole system in a heat shield having radiant heaters, which is heated to a modest temperature such as 150.degree. or 200.degree. C.
The difficulty with this is that many of the very numerous connections to the MBE system must be removed in order to perform this bake out. That is, an MBE system will have two or more liquid nitrogen connections to the low-jacket inside the growth chamber, will have electrical connections to monitor the temperature of the wafer and of the various molecular sources in place, will have optical microscopes or optical pyrometers or other optical accessories attached to viewports, etc. Since many of these connections must be removed to encase the MBE system temporarily in a heat shield, the prior art bake out is very time consuming and again greatly degrades throughput.
In the prior art, two methods are widely used for bakeout of an MBE system: most commonly, the growth chamber will be enclosed in a large heat shield, which contains radiant heaters mounted on the heat shield, to radiatively heat the whole MBE growth chamber. An alternative method is to locate a radiantly heater inside the MBE growth chamber, to radiately heat the system from the inside. However, this latter system has the disadvantage that the filament inside the system is itself likely to omit contaminants. In other vacuum systems, bakeout by resistive heaters attached to the walls of the vacuum chamber has in fact been done. This has not heretofore been considered practical for MBE systems, due to the problems of localized heating. No space for a heater mount can be found near the souces. Thus lateral thermal diffusion over long distances is required to heat the source as well as the rest of the vacuum chamber and this results in very long bakeout times.
However, the present invention is based on a realization that the requirements of partial bakeouts, performed periodically during routine operation, are different from the requirements of full bakeouts. During partial bakeouts, it is not necessary to bakeout the whole system evenly. The chief problem is due to material which has outgassed from the hot sources and condensed cold chamber walls, so that it is not the sources which need routine periodic bakeouts, but rather the chamber walls. Thus, contact heating can be used for these partial bakeouts, although use of a heat baffle with radiant heating is still necessary for complete bakeouts whenever the growth chamber has been exposed to the atmosphere.
Partial bakeout should be performed every week or two, whenever the background pressure of the system, while growth operations are not being performed, begins to rise. This indicates that the pump is becoming loaded by contaminants evaporating from the chamber walls, and a partial bakeout is desirable.
The present invention teaches an MBE system with an in-place bake out heater. That is, heating tape is preferably wrapped around the growth chamber and the analysis chamber, and left in place, in order to raise the temperature of those chambers to 120.degree. C. or so for periodic bakeouts. This permits frequent bakeouts, reducing the partial pressure of water vapor, hydrocarbons, and other undesirable species in the chamber vacuum, and thus enhances growth quality without degrading throughput.
A further problem of the prior art is that outgassing of the molybdenum substrate holder can never be adequately performed. That is, molybdenum, like many materials, forms a thin layer of native oxides upon contact with the atmosphere. These native oxides cannot be outgassed after a wafer has been mounted. Since a gallium arsenide wafer cannot be heated above about 650.degree. C. without degrading the surface quality, it is impossible to outgas the molybdenum adequately when it is in contact with the gallium arsenide wafer. In the prior art, this essentially means that molydenum substrate holders can never be fully outgasses at all, and will therefore always have native oxides in close proximity to the wafer. Since these native oxides are somewhat volatile, this provides a source of oxygen and oxide impurities which may be transported to the growth front to cause morphological defects in the wafer grown.
In a further embodiment of the present invention, the retaining ring used to affix the wafer to the substrate holder itself has pins on its edge for mechanical manipulation. This novel wafer support structure operates together with a vacuum wafer manipulation system, wherein wafers are loaded merely as a cassette of 10 or 20 wafers for MBE growth. After MBE growth has been performed on all of these wafers, the full cassette of wafers is then removed. However, the substrate holder and substrate retainer ring stay in the ultra high vacuum part of the system, and this reduces the number of parts which are cycles from ultra high vacuum into and out of atmospheric conditions. That is, the fewer the number of parts (or the smaller the amount of surface area) which is introduced to the ultra high vacuum growth chamber for each growth run, the cleaner the growth chamber can be kept. This is desirable. Moreover, if the number of cycles of even the secondary chamber up to atmospheric pressure can be reduced, system throughput can be further increased.
Thus, it is an object of the present invention to provide a molecular beam epitaxy system with increased throughput.
It is a further object of the present invention to provide a molecular beam epitaxy system wherein substrate holders need not be exposed to the atmosphere after each growth run.
It is a further object of the present invention to provide a molecular beam epitaxy system wherein only a cassette of wafers need be loaded into and removed from the vacuum system for each series of growth runs.
It is a further object of the invention to provide an MBE system wherein the substrate holder can be thoroughly outgasses after each exposure to atmosphere.
The present invention accomplishes these and other objectives by providing a high vacuum manipulation system which is preferably embodied in a system having horizontal wafer transport. A special wafer cassette is used which has a top shaped to mate with a special substrate holder. In the sample preparation chamber, the cassette is mounted on top of an elevator, i.e. a vertical in-vacuum linear motion element. A first fork arm is located so that it can transfer wafers between the substrate holder and the cassette when the substrate holder is located on top of a cassette. A second transfer fork is positioned so that it can lift the substrate holder from its position atop the cassette and transfer it. A retaining ring holder in the vacuum chamber is connected to a rotary and axial motion feedthrough, and can be used to screw on the retaining ring after the substrate holder has been transferred by the second transfer fork. Thus, the retaining ring holder contains slots which engage the pins on the retaining ring, so that the retaining ring holder can attach the retaining ring to the substrate holder and can remove the retaining ring from the substrate holder.
Thus, the apparatus can assemble a wafer to its mount in vacuum and load the wafer thus mounted in its substrate holder onto a transfer fork. As is conventional, the transfer fork then transfers the substrate holder and wafer into the growth chamber, wherein the substrate holder is attached to a conventional substrate support with rotary drive.
It should be noted that all of these operations can be completely automated. Thus, the present invention provides a MBE system wherein transfer of objects into and out of the ultra high vacuum growth chamber is minimized. The present invention also provides an MBE system wherein wafer loading and unloading onto the growth site can be automatically controlled.